[XeTeX] Getting completely the wrong glyphs when using characters from Latin Extended B

[Philip TAYLOR]

Michiel Kamermans wrote:

> The short answer is that \setmainfont is a fontspec command, and
> fontspec "knows what you want".

and Peter Dyballa wrote:

 > The quotes are needed when you use (Xe)TeX primitives
 > (\font\fontname...) with some automatisms attached to it, the quotes are
 > too much when you use fontspec. \setmainfont is a fontspec command.

OK, thanks Michiel & Peter.  But one last question (if I may ?) :
how does fontspec "know what I want" ?  Given a simple, unquoted,
name such as Optima, how can fontspec know whether I want to use
my Type-1 version or a TFM-based version ?

** Phil.